All living things are composed of cells which fall basically into two types eukaryotic, or prokaryotic. Bacteria are prokaryotic cells. Typically there is no nucleus, and the intracellular structure lacks organules.

Animals and plants are composed of Eukaryocytic cells, These cells contain a nucleus and are characterized by several different intra-cellular organules. This section deals mainly with animal eukaryocytic cells, although prokaryotic metabolism is touched on because of its importance in antibiotic action.

The thumbnail on the right shows the typical features of a cell as seen under with a light microscope using a standard stain. The chief features are a cell membrane, a granular cytoplasm, and a nucleus with in most cases a darker staining nucleous.

To see other features usually requires special stains, or an electron microscope.

The chief constituents of the cytoplasm are water, electrolytes, fats, carbohydrates and proteins. Water constitutes about 80% of the cell.

The chief electrolytes are potassium, phosphates sulfates, magnesium and bicarbonates. In smaller concentrations are sodium, calcium and chlorine. Sodium and chlorine are found outside the cell in the interstital fluid, in the same proportions as sea water, but less concentrated. It is hypothesized that the concentration seen probably represents the concentration of sea water when multicellular animals first evolved from single celled animals.

Fats include the fat soluble phospho-lipids and cholesterol. Phospholipids are of importance because they makeup the membrane of the cell wall, the nuclear membrane, and of the various organules found in the cell.

Carbohydrates include glucose and glycogen. Most cells have some glycogen - about 1%. Muscle cells have about 3%, and the liver up to 7%.

Apart from water, proteins are the next bigest constituent, between 15% and 20%. They fall into two categories, structural proteins and functional proteins. The structural proteins provide the skeleton of the cell, the functional proteins include numerous 'working' polypeptides including enzymes. The cell wall is studded with specialized proteins, indeed the cell wall is about 70% protein. Membranes of organules also have these specialized proteins in their wall. Many of these are concerned with transport of substances in and out of the cell.

In the next section we will look at the makeup and structure of the cell in more detail, and then we will look at how structure is married to function.

Transmission electron microscope image of a thin section cut through the developing brain tissue (telencephalic hemisphere) of an 11.5 day mouse embryo. This higher magnification image of "Embryonic brain 80415", shows an area of the luminal surface of the telencephalon, which has a junctional complex and pinocytotic vesicles. The junctional complex is divided into three types of junctions: 1) the most apical is the tight junction, which controls and/or restricts the movement of molecules across epithelial layers and helps maintain polarity, 2) the zonula adherens and 3) the desmosome, which is a spot junction. The pinocytotic vesicles are formed from coated pits in the plasma membrane and are involved in endocytosis.

Electron Micrograph of negatively stained en:Proximal convoluted tubule of Rat en:Kidney tissue at a magnification of ~55,000x and 80KV. This is a close-up of the en:Zonula occludens (en:Tight junction) out of a larger image of a Kidney Tissue with Tight junction.

Transmission electron microscope image of a thin section cut through the developing brain tissue (telencephalic hemisphere) of an 11.5 day mouse embryo. This image of the luminal surface of the telencephalon, shows junctional complexes and pinocytotic vesicles. The junctional complex is divided into three types of junctions: 1) the most apical is the tight junction, which controls and/or restricts the movement of molecules across epithelial layers and helps maintain polarity, 2) the zonula adherens, which also includes the numerous actin filaments seen in the apical cytoplasm, and 3) the desmosome, which is a spot junction. The pinocytotic vesicles are formed from coated pits in the plasma membrane and are involved in endocytosis.